In 3GPP LTE Rel-13 machine type communication (MTC) working item description (WID), there are three requirements, which are respectively low complexity, coverage enhancement, and power consumption. For coverage enhancement (CE), a target maximum coupling loss (MCL) of 155.7 dBm is proposed for both Rel-13 low complexity MTC UE and other non-MTC UE operating delay tolerant MTC applications. For PUSCH with the bottleneck of MCL among all physical channels, i.e. 140.7 dBm MCL, the target MCL of 155.7 dBm means maximum 15 dB coverage enhancement. Considering compensation of some coverage loss caused by low complexity, e.g. 3 dB transmission power reduction for uplink, it is required to achieve maximum 18 dB coverage enhancement. In order to achieve the target of coverage enhancement, repetition is necessary for most physical channels/signals. Many technologies can be used to improve the efficiency of repetition, i.e. to reduce the number of repetitions as far as possible, such as cross-subframe channel estimation, increasing DMRS density, uplink PSD boosting, and frequency hopping and so on.
In CE mode for Rel-13 MTC, channel repetition has been considered to improve data reception at low working SINR operation point by combining the repeated data. In the current LTE system, HARQ operation with one transmission per HARQ attempt has been used to exploit the frequency/time diversity of the channel for the combination gain. Besides, HARQ operation is also coupled with link adaptation, e.g., link adaptation may target 10% BLER after 1st HARQ transmission. Essentially, LTE HARQ transmission is a Stop-And-Wait (SAW) retransmission whereas the channel repetition is a non-stop or blind retransmission without feedback. So it seems necessary to support channel repetition as well as HARQ retransmission due to slightly different purposes. In this case, channel repetition can be applied for each HARQ transmission attempt while HARQ (re)transmission with feedback can be still kept. So HARQ mechanism should be supported with channel repetition for each HARQ transmission attempt.
Regarding to HARQ transmission for UE in CE mode, it seems not necessary to support full-adaptive HARQ, which may cause the large control overhead due to the data retransmission associated control channel carrying full control information and high power consumption for UE receiving the repeated control channel. Typically, the larger DCI the longer time for UE processing. However, the adaptation on the repetition number for retransmission is meaningful, which can improve spectrum efficiency by using a smaller repetition number than the repetition number for the initial transmission.
A new design to support HARQ retransmission for UE in CE mode is needed. In light of such motivation, a compact DCI with truncated format carrying partial control information for HARQ retransmission is proposed to carry the necessary and limited information for support the partial-adaptive HARQ for UEs in CE mode.
Improvements and enhancements are required for HARQ with channel repetitions.